We show how phase-space simulations of quantum states in a linear photonic network permit the verification of measurable probabilities and entanglement. We compare our predictions with recent Gaussian boson sampling experiments of Zhong et al. These use squeezed inputs and efficient “on-off” detectors, with up to 76th-order measured coincidence counts in the data. We introduce a general definition of grouped “on-off” detection probabilities for this purpose. The positive-$P$ phase-space method is used to compute any grouped or marginal click probabilities. Additional decoherence is included to obtain agreement between theory and experiment. The only limitation in estimating grouped probabilities is the computational sampling error, which is similar in magnitude to the experimental sampling error. The results obtained and graphed here are from first-order up to 16 000th-order grouped count probabilities. However, any order between these is also computable. We extend these results to include grouped probabilities with multidimensional outcomes that have a polynomial number of points. We also analyze quadrature detection experiments and show how to simulate genuine multipartite entanglement using Wigner phase-space methods.

The study of optimal control of quantum annealing by modulating the pace of evolution and by introducing a counterdiabatic potential has gained significant attention in recent times. In this work, we present a numerical approach based on genetic algorithms to improve the performance of quantum annealing, which evades the Landau-Zener transitions to navigate to the ground state of the final Hamiltonian with high probability. We optimize the annealing schedules starting from the polynomial ansatz by treating their coefficients as chromosomes of the genetic algorithm. We also explore shortcuts to adiabaticity by computing a practically feasible $k$-local optimal driving operator, showing that even for $k=1$ we achieve substantial improvement of the fidelity over the standard annealing solution. With these genetically optimized annealing schedules and/or optimal driving operators, we are able to perform quantum annealing in relatively short timescales and with higher fidelity compared to traditional approaches.

Vibrational ladder climbing (VLC), which is a cascade excitation process of molecular vibrational levels under chirped laser pulses, is one of the promising methods to realize molecular bond breaking, but there have been few experimental reports on bond breaking by VLC. In this study we observed vanishing of the transition dipole moment, interrupting VLC in general molecular systems. This phenomenon, that adjacent transition is forbidden at a certain vibrational level, was found to be explained by the parities of the dipole function and the vibrational wave functions. In order to preserve VLC, we proposed a method to use an additional chirped pulse that realizes “double-stepping” transitions at levels where adjacent transitions are forbidden. To show the effectiveness of our method, we conducted wave-packet dynamics simulations for LiH dissociations with chirped pulses. The results indicate that the efficiency of LiH dissociation is significantly improved by our method compared to conventional VLC methods with a single pulse. We also revealed the quantum interference effect behind the excitation process of VLC and found that the relative phase between the main pulse and the additional pulse has a significant effect on the dissociation probabilities of molecules.

We perform a dynamical finite-size scaling analysis of a nonequilibrium Bose gas which is confined in the transverse plane. Varying the transverse size, we establish a dimensional crossover for universal scaling properties far from equilibrium. Our results, which are based on classical-statistical real-time lattice simulations, suggest that some aspects of the dynamical universal behavior of anisotropic systems can be classified in terms of fractional spatial dimensions. We discuss our findings in view of recent experimental results with quasi-one-dimensional setups of quenched ultracold quantum gases.

We propose a transfer-matrix theory to reveal the hidden structure in the multipartite nonlocality operators in one-dimensional (1D) quantum chains. The theory offers a unified description for the scaling behaviors of multipartite quantum nonlocality in general 1D quantum lattices. Furthermore, in order to figure out the hierarchy of multipartite nonlocality for infinite-size chains, powerful transfer-matrix-based algorithms are proposed. In quantum critical regions, the algorithms converge much faster than the traditional approach.

Variational quantum algorithms have found success in the noisy intermediate-scale quantum era owing to their hybrid quantum-classical approach which mitigate the problems of noise in quantum computers. In our paper, we introduce the dynamic ansatz in the variational quantum linear solver for a system of linear algebraic equations. In this improved algorithm, the number of layers in the hardware efficient ansatz circuit is evolved, starting from a small and gradually increasing until convergence of the solution is reached. We demonstrate the algorithm advantage in comparison to the standard static ansatz by utilizing fewer quantum resources and with a smaller quantum depth on average in the presence and absence of quantum noise and in cases when the number of qubits or condition number of the system matrices are increased. The numbers of iterations and layers can be altered by a switching parameter. The performance of the algorithm in using quantum resources is quantified by a newly defined metric.

We explore the impact of non-Markovian channels on the quantum correlations of Haar uniformly generated random two-qubit input states with different ranks: Either one of the qubits (single-sided channel) or both qubits independently (double-sided channel) are passed through a noisy channel. For dephasing and depolarizing channels with varying non-Markovian strength, entanglement and quantum discord of the output states collapse and revive with the increase of noise. By both analytical and numerical means, we find that in the case of the depolarizing double-sided channel, entanglement and quantum discord for random states show a higher number of revivals on average than for single-sided channels with a fixed non-Markovianity strength, irrespective of the rank of the states; we call such a counterintuitive event, constructive feedback of non-Markovianity. In contrast, the mean value of critical noise at which quantum correlations (QCs) first collapse decreases with the increase of non-Markovianity, independently of the rank of the random initial states. However, the average noise at which QCs of random states show the first revival decreases with the increase of the strength of non-Markovian noise, thereby indicating the role of non-Markovian channels in the regeneration of QCs even in the presence of a high amount of noise. Moreover, we observe that the tendency of a state to show regeneration increases with the increase of average QCs of the random input states along with non-Markovianity.

Quantum chemistry applications on quantum computers currently rely heavily on the variational quantum eigensolver (VQE) algorithm. This hybrid quantum-classical algorithm aims at finding ground-state solutions of molecular systems based on the variational principle. VQE calculations can be systematically implemented for perturbations to each molecular degree of freedom, generating a Born-Oppenheimer potential-energy surface (PES) for the molecule. The PES can then be used to derive thermodynamic properties, which are often desirable for applications in chemical engineering and materials design. It is clear from this process that quantum chemistry applications contain a substantial classical computing component in addition to steps that can be performed using a quantum computer. In order to design efficient work flows that take full advantage of each hardware type, it is critical to consider the entire process so that the high-accuracy electronic energies possible from quantum computing are not squandered in the process of calculating thermodynamic properties. We present a summary of the hybrid quantum-classical work flow to compute thermodynamic properties. This work flow contains many options that can significantly affect the efficiency and the accuracy of the results, including classical optimizer attributes, number of ansatz repetitions, and how the vibrational Schrödinger equation is solved to determine vibrational modes. We also analyze the effects of these options by employing robust statistics along with simulations and experiments on actual quantum hardware. We show that, through careful selection of work flow options, nearly order-of-magnitude increases in accuracy are possible at equivalent computing time.

We report experimental and theoretical Bi $L_1$, $L_2$, and $L_3$ subshell ionization cross sections by the impact of electrons with energies from the Bi $L_3$ ionization threshold to 100 keV. The x-ray spectra have been acquired with two Si drift detectors placed in vacuum, which allowed us to better evaluate the peak fit procedure in the $L$ multiplet. The $L\alpha$, $L\beta$, $L\gamma$, $L\ell$, and $L\eta$ x-ray production cross sections, measured with relative uncertainties ranging from $5\%$ to $9\%$, and two sets of atomic relaxation parameters have been used to derive the Bi $L_1$, $L_2$, and $L_3$ ionization cross sections. Although the experimental uncertainties of the subshell ionization cross sections are smaller than those of the few previous measurements, they remain large due to the uncertainties associated with the relaxation parameters. Furthermore, ionization cross sections have been calculated for the three $L$ subshells with the subconfiguration average distorted-wave (SCADW) formalism, which includes the full two-body retarded electromagnetic interaction between the projectile and target electrons. These theoretical cross sections are $15\%$ to $30\%$ lower than the measured values, but the agreement is reasonable given the aforementioned high uncertainties. We have also found that the simpler distorted-wave Born approximation yields subshell ionization cross sections that match those computed with the SCADW method.

Dispersion relations are fundamental characteristics of the dynamics of quantum and wave systems. In this work we introduce a simple technique to generate a large variety of dispersion relations in a modulated tilted lattice. The technique is illustrated by important examples: the Dirac, Bogoliubov, and Landau dispersion relations (the latter exhibiting the roton and the maxon). We show that adding a slow temporal phase shift to the lattice modulation allows one to reconstruct the dispersion relation from dynamical quantities. Finally, we generalize the technique to higher dimensions and generate graphenelike Dirac points and flat bands in two dimensions.

Instabilities of vortex-ring-bright coherent structures in harmonically trapped two-component three-dimensional Bose-Einstein condensates are studied numerically within the coupled Gross-Pitaevskii equations and interpreted analytically. Interestingly, the filled vortex core with a sufficiently large amount of the bright component is observed to reduce the parametric interval of stability of the vortex ring. We have identified the mechanisms of several linear instabilities and one nonlinear parametric instability in this connection. Two of the linear instabilities are qualitatively different from ones reported earlier and are associated with azimuthal modes of $m=0$ and $m=1$, i.e., deviations of the vortex from the stationary ring shape. Our nonlinear parametric resonance instability occurs between the $m=0$ and $m=2$ modes and signals the exchange of energy between them.

Kerr optical frequency combs generated in a coherently driven Kerr nonlinear resonator have the potential for a wide range of applications. However, in a single cavity which is a widely adopted configuration for Kerr optical frequency-comb generation, modulation instability is suppressed in the normal dispersion regime, and the pump-to-comb conversion efficiency is extremely low for a single dissipative Kerr soliton (DKS) in the anomalous dispersion regime. Dual-coupled cavities have been proposed to generate Kerr optical frequency combs in the normal dispersion regime, and have the potential to remarkably increase conversion efficiency for Kerr optical frequency combs. Here, we investigate modulation instability and Kerr optical frequency-comb formation in dual-coupled cavities. Based on solutions of the continuous-wave steady state, we obtain a quadric algebraic equation describing the modulation instability gain, and we find that it is intensely influenced by the scaled free spectral range mismatch between the two cavities. Our numerical simulations demonstrate that platicons can be generated via a pump scanning scheme for the case that both the two cavities possess normal dispersion, and a single DKS can be generated in the cavity with anomalous dispersion while the dispersion of the other cavity is normal. The conversion efficiency of a single DKS is determined by the pump power and detunings as well as intrinsic quality factors of two cavities. Our analysis of modulation instability provides a powerful tool for Kerr optical frequency-comb generation via pump modulation and cavity detuning tuning scheme in dual-coupled cavities.

We report on the properties of a nonconventional stochastic photonic source. We first describe the principle of nonlinear intensity filtering by using a modified Mach-Zehnder interferometer. The latter alters the characteristics of an input stochastic source based on Bose-Einstein emission. Computed output intensity fluctuations are compared to an analytical model. Adjusting the interferometer parameters, we show theoretically and numerically that the statistical properties of light such as its probability density function can be tailored. Depending on the parameters, the probability density may exhibit large overshoots or smoother fluctuations. We further evaluate the impact of these modified statistics on simple nonlinear processes. Compared to Bose-Einstein emitters, the yield of nonlinear phenomena varies by several orders of magnitude. We finally simulate the nonlinear interaction of such a laser source with a dielectric material (fused silica) within the framework of a more realistic model, including a statistical analysis. It confirms that the deposited energy varies over many decades and can be largely enhanced due to the properties of the presented laser source.

Based on the combined Dicke, Bose-Hubbard, and Lipkin-Meshkov-Glick Hamiltonian, we investigate the formation of photonic rogue waves, including their Akhmediev and Kuznetsov-Ma cousins, in the strongly dispersive regime of a generalized atom-cavity array configuration. By means of the multiple-scale approach and mean-field approximation, we unveil that, due to the interplay between the photon hopping-induced tunneling effect and the self-attractive nonlinearity, the photons can redistribute in the whole array, which in turn leads to the redistribution (creation or annihilation) of excited atoms in each cavity and, consequently, the occurrence of Peregrine soliton states in both photonic and atomic fields. This interplay also yields an interesting intensity-clamping effect that the sum of the peak intensities of both rogue waves will be constant, although this quantity varies from site to site. We numerically confirm the analytical predictions, showing that the Peregrine rogue waves generated are stable against perturbations, despite the onset of modulation instability.

Mitigating errors is a significant challenge for near-term quantum computers. One of the most important sources of errors is related to the readout of the quantum state into a classical bit stream. A variety of techniques have been proposed to mitigate these errors with post-hoc corrections. We propose a complementary scheme to actively reduce readout errors on a shot-by-shot basis by encoding single qubits, immediately prior to readout, into multiqubit states. The computational resources of our technique are independent of the circuit depth and are compatible with the error rates and connectivity of many current devices. We analyze the potential of our approach using two types of error-correcting codes, and, as a proof of principle, we demonstrate an 80% improvement in readout error on the IBMQ Mumbai quantum computer.

Quantum resource theory formulations of thermodynamics offer a versatile tool for the study of fundamental limitations to the efficiency of physical processes, independently of the microscopic details governing their dynamics. Despite the ubiquitous presence of non-Markovian dynamics in open quantum systems at the nanoscale, rigorous proofs of their beneficial effects on the efficiency of quantum dynamical processes at the molecular level have not been reported yet. Here we combine the quantum resource theory of athermality with concepts from the theory of divisibility classes of quantum channels to prove that memory effects can increase the efficiency of photoisomerization to levels that are not achievable under a purely thermal Markovian (i.e., memoryless) evolution. This provides rigorous evidence that memory effects can provide a resource in quantum thermodynamics at the nanoscale.

Quantum key distribution (QKD) is theoretically secure using the principle of quantum mechanics; therefore, QKD is a promising solution for the future of secure communication. Although several experimental demonstrations of QKD have been reported, they have not considered the polarization-dependent loss in state preparation in the key-rate estimation. In this study, we experimentally characterized polarization-dependent loss in realistic state-preparation devices and verified that a considerable polarization-dependent loss exists in fiber- and silicon-based polarization modulators. Hence, the security of such QKD systems is compromised because of the secure key rate overestimation. Furthermore, we report a decoy-state BB84 QKD experiment considering polarization-dependent loss. Finally, we achieved rigorous finite-key security bound over up to 75 km fiber links by applying a recently proposed security proof. This study considers more realistic source flaws than most previous experiments; thus, it is crucial toward a secure QKD with imperfect practical devices.

Minimal, open quantum systems that are governed by non-Hermitian Hamiltonians have been realized across multiple platforms in the past two years. Here we investigate the dynamics of open systems with Hermitian or anti-Hermitian Hamiltonians, both of which can be implemented in such platforms. For a single system, the subject to unitary and thermal dynamics in a periodic manner, we show that the corresponding Floquet Hamiltonian has a rich phase diagram with numerous exceptional-point (EP) degeneracy contours. This protocol can be used to realize a quantum Hatano-Nelson model that is characterized by asymmetric tunneling. For one unitary and one thermal qubit, we show that the concurrence is maximized at the EP that is controlled by the strength of Hermitian coupling between them. Surprisingly, the entropy of each qubit is also maximized at the EP. Our results point to the multifarious phenomenology of systems undergoing unitary and thermal dynamics.

We propose a scheme for nonadiabatic noncyclic geometric quantum computation in a decoherence-free subspace (DFS). The physical system we used is a cavity quantum electrodynamics system consisting of nitrogen-vacancy (NV) centers in diamond and a whispering-gallery-mode (WGM) microresonator. This scheme not only has the feature of DFSs insensitive to decoherence, but also has the advantage of nonadiabatic noncyclic geometric quantum computation which has the built-in noise-resilience geometric feature and further enhances the robustness due to the ability beyond the limitation of the cyclic condition. An unconventional two-qubit phase gate in DFS has been realized in one step with our parallel protocol, which can largely shorten the computation time and the system error caused by decoherence. In addition, a scheme to realize a parallel process of two evolution paths is put forward to achieve the distribution quantum computation of two WGM microresonators containing a NV center in each. Our scheme may provide a promising way towards the practical realization of high-fidelity geometric quantum computation with the features of a DFS, geometric phase, and parallel implementation process.